Both long term and batch experiments were carried out to identify the sources of the N2O emission in anoxic/aerobic sequencing batch reactors (A/O SBRs) under different aeration rates. The obtained results showed that aeration rate has an important effect on the N2O emission of A/O SBR and most of the N2O was emitted during the aerobic phase. During the anoxic phase, nitrate ammonification was the major source of N2O emission while denitrification performed as a sink of N2O, in all three bioreactors. The N2O emission mechanisms during the aerobic phase differed with the aeration rate. At low and high aeration rates (Run 1 and Run 3), both coupled-denitrification and nitrifier denitrification were ascribed to be the source of N2O emission. At mild aeration rate (Run 2), nitrifier denitrification by Nitrosomonas-like ammonia oxidizing-bacterial (AOB) was responsible for N2O emission while coupled-denitrification turned out to be a sink of N2O because of the presence of inner anaerobic region in sludge flocs. (C) 2011 Elsevier Inc. All rights reserved.